In the manufacture of footwear of various types, it is often a problem to develop a product having all the physical properties which are desired. A leather sole thick enough to protect the foot is expensive and often is too heavy for wearing comfort. A rubber sole shoe made of ordinary rubbers requires vulcanization and has a number of properties which are found to be disadvantageous in footwear.
Recently, synthetic rubber has begun to replace polyvinyl chloride, natural rubber, and leather for casual shoe soles and heels. These synthetic rubbers, such as the styrene-diene block copolymers of Hendricks et al. U.S. Pat. No. Re. 28,236, have a number of advantages. For one, these block copolymer compositions do not require a vulcanization step. Further, the scrap from these block copolymer shoes may be recycled since these polymers are clearly "thermoplastic" elastomers.
In order to reduce the weight and cost of the shoe sole and heel (unit sole), and to improve the cooling and cycle time involved in producing the unit sole, it has often been necessary to employ molds that result in a hollow core or cavity in the shoe heel. However, one of the major problems with injection molded unit soles is that the method of fabrication and its material of construction produce a hollow core module that does not generally provide sufficient structural rigidity to support the various heel heights or the shank area of today's modern casual shoe. Thus, considerable flexing, bulging and shoe deformation can and does occur in these critical load areas of the shoes when worn.